This invention relates, in general, to manufacturing olefins from lower alkanes. As used herein, the term xe2x80x9colefinsxe2x80x9d means ethylene, propylene, butenes, pentenes, hexenes and higher olefins. The term xe2x80x9clower alkanesxe2x80x9d means methane, ethane and/or propane. More particularly, the present invention relates to methods for manufacturing olefins such as ethylene and propylene from methane, ethane, and/or propane by oxidative dehydrogenation at elevated pressure, wherein the olefins are recovered from unconverted methane, ethane, and/or propane and reaction byproducts by using a complexation separation. In one embodiment of the invention, recycle of reaction byproducts is reduced or eliminated by adding an effluent containing unconverted methane, ethane, and/or propane and reaction byproducts to a methane gas transport system, such as a natural gas pipeline.
Methane is an attractive raw material because it is widely available and inexpensive; however, it is used mainly as a fuel. Natural gas liquids, such as ethane and propane, are the major raw materials for the production of ethylene and propylene, from which many petrochemicals are produced. But the supply of natural gas liquids has not kept pace with increasing demand for olefins, so more costly cracking processes that use naphtha from petroleum are being commercialized. Therefore, the development of economical processes for manufacturing olefins from methane and other lower alkanes is highly desirable.
Methane has low chemical reactivity, so severe conditions are required to convert it to higher hydrocarbons such as olefins. Oxidative dehydrogenation is favored because conversion is not thermodynamically limited and reactions are exothermic. But selectively producing ethylene by partial oxidation, while avoiding over-oxidation to carbon oxides, has been elusive and is difficult to achieve. Therefore, since the first screening of oxidative dehydrogenation coupling catalysts was reported by G. E. Keller and M. M. Bhasin in xe2x80x9cSynthesis of Ethylene via Oxidative Coupling of Methane. I. Determination of Active Catalystsxe2x80x9d, Journal of Catalysis 73: 9-19 (1982), great effort has been made to develop selective catalysts and processes for methane coupling.
Catalyst studies have nearly all been at atmospheric pressure, with only a few studies conducted at elevated pressure. This is the case because it has been reported that increasing pressure reduces coupling selectivity, primarily due to increased homogeneous or heterogeneously catalyzed combustion. The oxidative dehydrogenation coupling reaction is highly exothermic, and a high reaction temperature is usually generated within a hot spot after the reactants are heated to the initiation temperature. The temperatures employed generally exceed 650xc2x0 C. and are typically 800 to 900xc2x0 C. An important catalyst characteristic is lifetime, especially under such high temperature conditions. Sustained operation at excessively high temperatures usually causes significant to substantial decay in selectivity and may also result in the loss of catalytic and promoter components through slow-to-rapid vaporization.
Process studies have developed cofeed (continuous) processes and sequential (pulsed) processes. The cofeed processes pass methane and oxygen simultaneously over a catalyst in a fixed-bed or fluidized-bed reactor. They typically use low methane conversion for safety and because olefin selectivity decreases as conversion increases. The reactions are operated under oxygen-limited conditions, i.e., very high or total oxygen conversion. The sequential processes alternately contact the catalyst with oxygen (oxidation) and then methane (reduction), either in cyclic pulses or in separate reactors. Because methane does not contact gaseous oxygen in sequential processes, homogeneous oxidation is suppressed, and conversion can be higher.
Sequential catalysts are typically reducible metal oxides that function as oxygen transfer agents. Materials that have been used as sequential catalysts include a wide variety of reducible metal oxides, mixed metal oxides and other reducible compounds of the following metals: Sn, Pb, Bi, Tl, Cd, Mn, Sb, Ge, In, Ru, Pr, Ce, Fe, Re, Tb, Cr, Mo, W, V or mixtures thereof. Promoters include oxides or compounds containing alkali metals, alkaline earth metals, boron, halogens, Cu, Zr, or Rh. Processes which utilize a reducible metal oxide catalyst are disclosed, for example, in the following references: U.S. Pat. No. 4,547,607 discloses methane coupling wherein a portion of the C2+ alkanes recovered are subsequently recycled to the reactor. No examples under pressure are given. U. S. Pat. No. 4,554,395 discloses methane coupling at elevated pressure (100 psig and 700xc2x0 C.) to promote formation of C3+ hydrocarbons, but does not disclose the effect on C2 hydrocarbons. The higher C3+ selectivity decreases considerably after just a few minutes. U.S. Pat. No. 4,754,093 discloses reacting methane and air, adsorbing higher hydrocarbons on activated carbon at atmospheric pressure, selectively desorbing olefins under vacuum, and recycling higher alkanes with the uncoverted methane.
Many metal oxides, carbonates, and promoted mixtures, often supported on substrates such as alumina, silica, and titania, have been used as cofeed catalysts for oxidative dehydrogenation coupling. These include alkaline earth metal oxides, alkali metal oxides or halides, and oxides of Mn, Co, Ni, Zn, Bi, Pb, Sb, Sn, Tl, In, Cd, Ge, Be, Ca, Sr, Ba, Sc, Y, Zr, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, or Lu, either individually or as mixtures thereof. Other metal-containing materials such as various zeolites have also been used. The metal oxides are often promoted with alkali metals and/or alkaline earth metals or their oxides, halides, or carbonates. Basic oxides promoted with alkali metal carbonates are important catalysts, as well as transition metal compounds.
Cofeed catalysts and oxidative dehydrogenation coupling processes utilizing such catalysts are disclosed, for example, in the following references: U.S. Pat. Nos. 4,695,668 and 4,808,563 disclose catalysts containing Mo-W compounds, which gave C2 and oxygenated hydrocarbons, and much CO, at 520 to 800 psig. U.S. Pat. Nos. 5,066,629 and 5,118,898 disclose separating natural gas into methane and higher alkanes, oxidatively coupling the methane, pyrolyzing the higher alkanes by using the heat released, cryogenically separating the combined products, and recycling recovered ethane to the pyrolysis reaction. Integrated processes for converting natural gas into higher hydrocarbons are further disclosed in U.S. Pat. Nos. 5,025,108; 5,254,781; 5,736,107; and 5,336,825. The latter discloses recycling, to the coupling reaction, which is preferably done at 1-2 atmospheres pressure, the methane and C2 hydrocarbons left over from subsequently converting the olefins to liquid hydrocarbons. Note also, J. L. Matherne and G. L. Culp, xe2x80x9cDirect Conversion of Methane to C2""s and Liquid Fuels: Process Economicsxe2x80x9d, pages 463-482, in E. E. Wolf, Methane Conversion by Oxidative Processes, Fundamental and Engineering Aspects, Van Nostrand Reinhold (1992).
A number of these prior art references disclose recycling unconverted methane containing byproduct alkanes to the oxidative dehydrogenation coupling reaction. These references suggest that the reaction may be done under elevated pressure, but they do not demonstrate that recycling such a composition is feasible or beneficial when the reaction is done under elevated pressure. Furthermore, the aforementioned processes that demonstrate conducting the oxidative dehydrogenation coupling reaction under elevated pressure do not suggest or demonstrate recycling unconverted methane containing byproduct alkanes to the reaction. The aforementioned processes also disclose cryogenic distillation separation, adsorption/desorption separation using activated carbon or charcoal, and separation by subsequent olefin reaction as methods by which olefins may be separated from unconverted methane and byproduct alkanes, but they do not disclose using complexation separation methods.
Several literature studies have found that operating the oxidative dehydrogenation coupling reaction under elevated pressure reduces C2 selectivity and/or catalyst activity. G. J. Hutchings, et al., xe2x80x9cThe Role of Gas Phase Reaction in the Selective Oxidation of Methanexe2x80x9d, Journal of the Chemical Society, Chemical Communications 1988: 253, found that C2 selectivity was higher without using a Li/MgO catalyst at 85 psi. A. Ekstrom, et al., xe2x80x9cEffect of Pressure on the Oxidative Coupling Reaction of Methanexe2x80x9d, Applied Catalysis 62: 253 (1990), found that increasing pressure to 87 psi depressed C2+ selectivity and catalyst activity for Li/MgO, Sm2O3, and SrCO3/Sm2O3 catalysts, by increasing uncatalyzed combustion. M. Pinabiau-Carlier, et al., xe2x80x9cThe Effect of Total Pressure on the Oxidative Coupling of Methane Reaction Under Cofeed Conditionsxe2x80x9d, pages 183-190 in A. Holmen, et al., Studies in Surface Science and Catalysis, 61, Natural Gas Conversion, Elsevier Science Publishers (1991), found that increased pressure decreased C2+ selectivity for a strontium-doped lanthanum oxycarbonate catalyst, and recommended operating at pressures below 3 bar (43.5 psi).
It is known that some metal ions, primarily silver or copper salts, complex selectively and reversibly with olefins, and therefore they can be used to recover olefins from hydrocarbon mixtures by absorption, adsorption, or membrane separation methods. A variety of complexation agents have been developed. However, the use of complexation based separations for large-scale olefin recovery has been limited to proposals for olefin recovery in petroleum refining operations or gas-cracking olefin plants, or to purify ethylene from ethane or propylene from propane. Membranes are only suitable for small-scale recovery of olefins, such as from vent gases. Examples of such prior art complexation-based separations are disclosed in U.S. Pat. Nos. 4,174,353 and 5,859,304, and in R. B. Hall and G. R. Myers, xe2x80x9cEffects of Product Separation on the Kinetics and Selectivity of Oxidative Couplingxe2x80x9d, pages 123-130 in M. M. Bhasin and D. W. Slocum, Methane and Alkane Conversion Chemistry, Plenum Press (1995), and E. M. Cordi, et al., xe2x80x9cSteady-State Production of Olefins in High Yields During the Oxidative Coupling of Methane: Utilization of a Membrane Contactorxe2x80x9d, Applied Catalysis A: General 155: L1-L7 (1997).
Clearly, there is a need for improved methods for producing olefins from methane and other lower alkanes by oxidative dehydrogenation that are both economical and suitable for large-scale production. Such methods would utilize optimal reaction conditions, have few process steps, and be highly effective in recovering the olefin product, despite the olefin being present in low concentration in the reactor effluent, due to the typically low single-pass conversion that is characteristic of oxidative dehydrogenation. Such methods would avoid costly cryogenic separation of the dilute olefin products from the unconverted lower alkanes. In particular, such methods would be able to utilize the process advantages of carrying out the reaction at elevated pressure instead of at atmospheric pressure. They would also minimize the processing and disposal of reaction byproducts.
The present invention meets the above-noted objects by providing methods by which olefins, including ethylene, propylene, butenes, pentenes, hexenes and higher olefins can be produced economically and on a large-scale by the oxidative dehydrogenation of lower alkanes, i.e., methane, ethane and/or propane. The methods utilize optimal reaction conditions of elevated pressure and lower temperature, which increases catalyst stability. This is facilitated by using catalysts having performance characteristics favorable for reaction at elevated pressure. Process steps are minimized, which reduces cost and capital investment. In the case of methane, natural gas can be used as a methane source without first removing higher hydrocarbons. Byproducts such as ethane, propane, and hydrogen need not be separated from the unconverted methane or the recovered olefins. Yet the purge stream can be a small fraction of the recycle. The oxidative dehydrogenation reaction need not be integrated with other reaction steps, such as cracking byproduct ethane, which is typically run at atmospheric pressure. The olefin products are recovered selectively and efficiently from the unconverted methane, ethane, and/or propane, despite being present in low concentration, without using costly cryogenic separation. And the olefin recovery can be done at elevated pressure, thereby minimizing energy losses during decompression and compression as the gas pressure is lowered and raised, respectively. The olefin products can be readily separated with high purity.
In one embodiment, the method taught by the invention for producing olefins from one or more lower alkanes by oxidative dehydrogenation comprises the steps of: (1) supplying at least one lower alkane; (2) providing a source of oxygen; (3) converting a portion of the lower alkane by an oxidative dehydrogenation reaction process that utilizes a catalyst, to produce unconverted lower alkane containing at least one olefin product, at least one alkane byproduct, and water, wherein the reaction pressure is at least about 50 psi and olefin product(s) and alkane byproduct(s) are formed from the lower alkane with a combined selectivity of at least about 40%; (4) removing water from the unconverted lower alkane; (5) recovering the at least one olefin product from the unconverted lower alkane by using a complexation separation that utilizes at least one complexation agent to selectively remove olefins from non-olefins and which is not a membrane separation; and (6) recycling after steps (4) and (5) a majority of the unconverted lower alkane which contains the at least one alkane byproduct to the oxidative dehydrogenation reaction process of step (3).
In another embodiment, a method taught by the invention for producing ethylene and/or propylene from one or more lower alkanes by oxidative dehydrogenation comprises the steps of: (1) supplying at least one lower alkane; (2) providing a source of oxygen; (3) converting a portion of the lower alkane by an oxidative dehydrogenation reaction process that utilizes a rare earth oxycarbonate catalyst, to produce unconverted lower alkane containing at least ethylene and/or propylene, at least one alkane byproduct and/or higher olefin, and water, wherein the reaction pressure is at least about 75 psi and olefin(s) and alkane byproduct(s) are formed from the lower alkane with a combined selectivity of at least about 40% and a mole ratio of olefin(s) to alkane byproduct(s) of at least about 1/1; (4) removing water from the unconverted lower alkane; (5) recovering ethylene and/or propylene from the unconverted lower alkane by using an aqueous complexation absorption separation that utilizes at least one complexation agent containing a silver (I) ion to selectively remove ethylene and/or propylene from higher olefins and non-olefins and which is not a membrane separation; and (6) recycling after steps (4) and (5) a majority of the unconverted lower alkane which contains the at least one alkane byproduct and/or higher olefin to the oxidative dehydrogenation reaction process of step (3).
In still another embodiment, the method taught by the invention for producing olefins from one or more lower alkanes by oxidative dehydrogenation, wherein recycling of unconverted lower alkane containing reaction byproducts is reduced or eliminated, comprises the steps of: (1) supplying at least one lower alkane; (2) supplying oxygen; (3) converting a portion of the lower alkane by an oxidative dehydrogenation reaction process that utilizes a catalyst, wherein the reaction pressure is at least about 50 psi, to produce unconverted lower alkane containing at least one olefin product, at least one combustible byproduct, and water; (4) removing water from the unconverted lower alkane; (5) recovering the at least one olefin product from the unconverted lower alkane by using at least one complexation separation that utilizes a complexation agent to selectively remove olefins from non-olefins and which is not a membrane separation; and (6) adding after steps (4) and (5) a majority of the unconverted lower alkane which contains the at least one combustible byproduct to a methane gas transport system. In a preferred embodiment, the lower alkane in step (1) is processed natural gas supplied from a natural gas transport system, such as a natural gas pipeline, and the methane gas transport system of step (6) is the natural gas transport system of step (1).
In yet another embodiment, the invention is a method for manufacturing olefins from one or more lower alkanes by oxidative dehydrogenation in which a gaseous effluent having substantially the same heating value as natural gas and containing at least one reaction byproduct is added to a natural gas transport system.